A super secretive US government satellite SpaceX launched in January never made it to orbit after it failed to separate from the upper stage of a Falcon 9 rocket. Three months later, it still appears the satellite manufacturer Northrop Grumman may be to blame for the loss and not SpaceX.

A new report from The Wall Street Journal published late Sunday says two teams of investigators have found that a payload adapter, which was modified by Northrop Grumman to accommodate the reportedly sensitive spy satellite, is the culprit behind the loss of the $3.5 billion craft.

The Journal report cites anonymous sources familiar with the federal government and industry investigations.

"Normally when you buy a rocket launch, you've paid for the payload adapter on the rocket final stage that pops the satellite off at the end,'" wrote astronomer and veteran launch observer Jonathan McDowell on Twitter in January. "But on this mission the customer provided its own payload adapter, so separation may be its problem and not SpaceX's problem."

So Much SpaceX

According to the Journal report, the adapter Northrop Grumman provided had to be modified for Zuma. And although it worked when tested three times on Earth, it failed to operate properly in the microgravity environment of space. The payload did eventually separate from the Falcon 9 upper stage, but by this point the rocket and satellite had already fallen too far back toward the atmosphere and an ultimately fiery demise.

SpaceX declined to provide an updated comment on the Journal report and the unfortunate end of the Zuma mission.

In January, President and COO Gwynne Shotwell said that "after review of all data to date, Falcon 9 did everything correctly on Sunday night ... If we or others find otherwise based on further review, we will report it immediately."

That was the last word from Elon Musk's rocket company on the matter, and it looks as though it's going to stay that way. Since then, the company has soldiered on, raising its profile with its huge Falcon Heavy launch system and a handful of other successful missions. Up next, it'll launch the Transiting Exoplanets Survey Satellite on April 16.

Northrop Grumman has also continued work on other big projects like the James Webb Space Telescope, which has recently been pushed back to a 2020 launch due to engineering and testing delays.